(a) Field of the Invention
The present invention relates to a hold display unit (hold type display unit) for display of a moving picture and, more particularly, to a hold display unit such as a hold LCD unit for display of a moving picture. The present invention also relates to a monitor, a light valve and a projector using the hold display unit.
(b) Description of the Related Art
Recently, a twist nematic (TN) mode LCD device is generally used as a typical LCD device. The TN mode LCD devices are categorized into two modes: an active matrix mode, such as TN-TFT, wherein a thin-film-transistor (TFT) switch is provided in each of the pixels of the display unit; and a super twisted nematic (STN) mode. Although the STN mode has improved characteristics as to a contrast and a viewing angle dependency over the TN-TFT mode, it has the disadvantages of a lower-speed response. Thus, the STN mode display unit is not suited to display of a moving picture. The STN mode also has the disadvantage of a poor image quality compared to the TN-TFT mode, which is now more used in the commercial base.
In the circumstances as described above, techniques for improvement of the viewing angle dependency have been developed and are now used in the practical products. Thus, the main stream of the high-performance LCD device uses a TN mode in association with a compensation film, an in-plane switching mode, and a TFT active matrix mode using a multi-domain vertical-aligned technique.
In the active matrix mode LCD devices as described above, the image signal is updated at a cycle of 60 Hz, for achieving positive and negative updating each at a cycle of 30 Hz, whereby a single field has about 16.6 milliseconds. Thus, the sum of the positive and negative fields, called a frame, has about 33.3 milliseconds. It is to be noted that the response speed of the current LCD devices resides around this frame time at most. Thus, the LCD devices are requested to achieve a response speed higher than that achieving this frame time if the LCD devices are used for display of image signals such as for moving pictures, computer graphics or high-speed game pictures.
A variety of techniques have been studied for achieving a high-speed mode of the LCD devices. The techniques for obtaining a higher-speed operation for LCD devices are categorized in two main streams including one directed to using a higher-speed nematic liquid crystal (LC) as described above and the other directed to using a smectic LC having a spontaneous polarization and a higher response characteristic.
The first stream directed to the higher-speed nematic LC attempts the techniques of: reducing the cell gap to increase the electric field per applied voltage; applying a higher voltage to the LC layer to increase the electric field, thereby promoting or assisting the state change of the LC layer; reducing the viscosity of the LC; and employing a specific mode which is considered to inherently achieve a higher speed. By using these techniques, a current response time of several milliseconds has been achieved for the LCD units.
As such examples, there are a field-sequential display mode, and optically-compensated birefringence mode, which achieve response times between 2 to 5 milliseconds. Such techniques are described in “Electronic Technology” from Nikkan Kogyou News Paper, July 1998, pp 8–12, and “SID '94 Digest” in pp 927–930. By using these techniques, response times between 2 and 5 milliseconds have been achieved.
Examples of the smectic LCs having a spontaneous polarization in the second stream include surface stabilized ferroelectric liquid crystals (SSFLC), which is most popular among them and used in practical products. The SSFLC is reported to have a response time of about 100 microseconds (μs). A similar response time is also obtained by an anti-ferroelectric LC having three stable states. In addition, modes using deformed helix ferroelectric LC, non-threshold anti-ferroelectric LC, and LC using an electroclinic effect also achieve higher response times between several milliseconds and several tens of microseconds in an analog display format.
However, it is reported that these higher-speed LCs cannot also display moving pictures with a sufficient image quality. This is considered due to the display principle itself of the LCD unit. It is to be noted that display units other than the LCD unit, such as a CRT unit, emits own light for the display by self-luminescence, whereas the LCD unit displays images by using a shutter function of the LC layer which transmits or blocks the light that is incident thereto by transmission or reflection.
In the operation of the CRT unit, the electron beam is irradiated to a phosphor for fluorescence. The lifetime of the fluorescent member depends on the phosphor and the objective of the. CRT unit. For example, in the long-persistence oscilloscope such as for radar, a phosphor is generally used which has a longtime fluorescence as long as several hundreds of milliseconds, during which the intensity of light reduces down to 10% of the original light. On the other hand, in a flying-spot scanning tube, a phosphor is generally used which has a short-time fluorescence as short as 100 nanoseconds. In a CRT unit used for display of moving pictures, a phosphor having a short-time fluorescence is used.
FIG. 1 shows a timing chart of the luminance of such a CRT unit for display of moving pictures in each field, wherein the luminance is higher only for an initial duration of the each field and reduces abruptly in the following duration of the each field, showing an impulse type luminance.
On the other hand, the shutter mode of the LCD device allows the luminance to be constant in each field to obtain a hold type luminance, as shown in FIG. 2, In FIG. 2, the solid line shows the case of an ideal high-speed response whereas the dotted line shows the case of a practical lower-speed response, illustrating the hold type luminance.
The impulse type luminance and the hold type luminance are examined for their display performances in the literatures such as proceedings of LCD Forum meeting, entitled “For LCD unit to replace CRT monitor market in the moving picture view point”, Aug. 8, 1998, pp 1–6, and a material of 62nd Joint Society meeting, Nov. 20, 1998, pp 1–5, held by division of Intelligent Organic Material of LC material, in 142 Committee of Organic Material Division of Jpn. Science Promotion Institute. These literatures include illustrations of the impulse type display and the hold display, showing how the moving character is observed differently therebetween. The illustrations are incorporated herein and shown as FIGS. 3A and 3B after miner modifications.
FIGS. 3A and 3B each shows the results of observation of the moving picture on the screen by a human eye, wherein character (or object) “A” moves in the direction of arrow, i.e., rightward direction. FIGS. 3A and 3B correspond to a CRT unit and a LCD unit, respectively.
On the CRT unit, as shown in FIG. 3A, the character A appears on a first location of the screen at an instant, disappears at the next instant, again appears at the next time on a second location apart from the first location, and again disappears at the next instant. On the LCD unit having a higher-speed response, as shown in FIG. 3B, the character A appears on a first location of the screen, stays at the first location until a next scanning period, moves abruptly from the first location to a second location at the next scanning period, and stays at the second location until a further next scanning period.
When the character A is traced by the human eye along the movement thereof on the CRT unit, as shown in FIG. 3A, the character is observed only at the luminescence thereof by the human eye, which tends to trace the character while moving at a constant speed. This allows a natural movement of the character. On the other hand, when the character is traced by the human eye along the movement thereof on the LCD unit, as shown in FIG. 3B, the character is observed for a while at the first location by the human eye, which tends to trace the character while moving at a constant speed. This causes the character to be observed as if the character moves on the retina of the human eye toward the leftward direction opposite to the moving direction of the character. Thus, the character is observed to have a tail, which hinders the character from being observed clearly.
In the analysis of observation by the human eye, it is noted that improvement of the response time alone is not sufficient for achieving suitable display of moving pictures by the hold LCD unit, and that the improvement should accompany specific holding schemes. The specific holding schemes are considered to include reduction of the hold time of the luminescence, and a configuration that the luminescent light is located in the vicinity of the locus of the movement of the character.
The reduction of the hold time can be achieved by a technique wherein a backlight source is periodically switched on and off in a high-speed LCD unit having a pi-cell structure using a compensation plate. This technique is described in the proceedings of the Forum of LCD Institute as described above, pp 20–23. Another technique for reduction of the hold time is such that the backlight source is normally turned on, with a reset state inserted therein. Such reduction is also described in the same proceedings of the Forum of the LCD Institute, pp 5–6.
As described above, in summary, the first problem in the prior art is that the hold LCD unit inherently degrades the image quality of the moving picture.
The second problem is that the shutter mode such as periodical switching or reset of the back light necessitates a complicated structure and yet achieves a limited effect, because sufficient improvement is only achieved by a longer dark time inserted therein. For example, for obtaining a display performance in the LCD unit comparable to the performance of the CRT unit, a single field should include a 1-millisecond-long bright time and a remaining dark time. In the periodical switching of the backlight, the drive circuit having a high driving voltage for the backlight is difficult to operate with a higher frequency without raising the costs thereof. On the other hand, in the reset of the backlight, a sufficient luminance is only achieved by a high-speed response of the LCD layer.